Drawbead structures are used to control the flow of a sheet metal blank within a drawing die during drawing operations. Generally such structures include a bead or male drawbead structure on the binder face of one die and a groove or female drawbead structure in the binder face of the other die. The drawbead structures interfit in spaced relationship to each other when the die is closed. The drawbead structure resists pulling of the edge portion of the sheet metal blank into the die through a combination of deformation forces and friction forces resulting from the edge portion of the blank being alternately bent and straightened as it passes around the bead and through the groove into the die.
Lockbead structures are used to prevent the flow of the sheet metal blank within a die during stretch forming operations. Generally such structures include a bead or male lockbead structure on the binder face of one die and a groove or female lockbead structure in the binder face of the other die. The lockbead structures interfit in closely spaced relationship to each other when the die is closed to clamp the sheet metal blank therebetween and prevent the edge portion of the blank from being pulled into the die.
Drawbead structures and lockbead structures are conventionally built on and into the dies, and during die tryout, the restraining force is set by adding or grinding off material from the bead and groove in a trial and error process.
It is often difficult, if not impossible, to determine in advance whether a die formed part can be successfully stretch formed or whether it must be drawn. If the die is designed with a lockbead structure and the part cannot be stretch formed without splitting, the bead and the groove must then be barbered or ground in a trial and error process to alter the lockbead structure into a drawbead structure. Such a process can be time consuming and often result in a poor drawbead structure if too much material is removed from the bead or groove. Likewise, if the die is designed with a drawbead structure and it is decided that it may be advantageous to form the part by a stretch forming operation, material must be added to the bead and/or the groove and some grinding must be done on a trial and error basis to convert the drawbead structure into a lockbead structure.